I think the intention is that the content of that folder shouldn't be modified by a style (as it doesn't make sense). Extensions may or may not modify it, but i don't really see the point of it in case of the feed template.
So IMHO it aims to make the style authors life a bit easier, not to restrict anything.

Thanks, is this folder destined not to be changed by any extension or so on? To last forever as it is?

Its basically a main parent style, parent of any installed style.

That particular template doesn't depend on any other parts of style, placing it in "all" directory makes it usable for any style. So whatever style you install on your forum, including styles that don't inherit from prosilver, that template will always be available.

Thanks, is this folder destined not to be changed by any extension or so on? To last forever as it is?

Its basically a main parent style, parent of any installed style.

That particular template doesn't depend on any other parts of style, placing it in "all" directory makes it usable for any style. So whatever style you install on your forum, including styles that don't inherit from prosilver, that template will always be available.